Interpretive Summary: Bacterial blight, caused by Xanthomonas oryzae pv. oryzae (Xoo), is one of the most important and devastating diseases of rice worldwide. In this study, a collection of more than 637 plant-associated bacteria isolated from the roots of crops grown throughout India were tested for the ability to inhibit the blast pathogen in laboratory assays and on rice grown in nethouse and field studies. Biochemical and genetic analyses of the mechanisms involved in inhibition of the pathogen by one isolate, PTB 9, revealed that the strain produced the well-known antibiotic 2,4-diacetylphloroglucinol (DAPG). To determine whether DAPG was responsible for the inhibitory activity of the strain, mutants that no longer produced the antibiotic were generated and compared to the parental strain for the ability to suppress blast on rice. The mutants were significantly less effective, implicating DAPG as a key factor in the activity of PTB 9. The results are significant because this is the first time that a DAPG-producing strain has been identified in India, and the first report of DAPG controlling a bacterial pathogen.

Technical Abstract:
Certain plant-associated strains of fluorescent Pseudomonas spp. are known to produce the antimicrobial antibiotic 2,4-diacetylphloroglucinol (DAPG). It has antibacterial, antifungal, antiviral, and antihelminthic properties and has played a significant role in the biological control of tobacco, wheat, and sugar beet diseases. It has never been reported from India and has not been implicated in the biological suppression of a major disease of the rice crop. Here, we report that a subpopulation of 27 strains of plant-associated Pseudomonas fluorescens screened in a batch of 278 strains of fluorescent pseudomonads produced DAPG. The DAPG production was detected by a PCR-based screening method that used primers Phl2a and Phl2b and amplified a 745-bp fragment characteristic of DAPG. HPLC, H-1 NMR, and IR analyses provided further evidence for its production. We report also that this compound inhibited the growth of the devastating rice bacterial blight pathogen Xanthomonas oryzae pv. oryzae in laboratory assays and suppressed rice bacterial blight up to 59%-64% in net-house and field experiments. Tn5 mutants defective in DAPG production (Phl(-)) of P. fluorescens PTB 9 were much less effective in their suppression of rice bacterial blight.